(1) Field of the Invention
The present invention relates to an optical modulation element module comprising an optical modulation element being integrated into a case, and in particular to an optical modulation element module which can be stably used especially in a hot environment.
(2) Related Art Statement
At present, optical modulation element is frequently used as one of the means for generating a light signal in the field of optical communication. The operating environment is normally in the range of −10° C. to 75° C. and the storage temperature is in the range of −40° C. to 85° C. for such optical modulation element for communication. In addition, the optical modulation element is assured of operating in that temperature range and of not being damaged during storage.
The optical modulation element has been used in various environments in recent years. Even when, for example, it is necessary to put a signal source in a hot environment such as a remote sensing technique like the underground resources exploration or the seismic layer exploration, needs for use of the optical modulation element has been appearing.
For the remote sensing technique for investigating the underground status, a hole for exploration 1 is dug, and a sensor 7 detects information such as pressure, temperature, mineral, gas or the like under the ground as shown in FIG. 1. An optical modulation element 4 is positioned adjacent to sensor 7 in order to send the detected information by sensor 7 in the form of a light signal. A light source 3, an optical detector 9 etc. are positioned on the ground in order to gain highly accurate information avoiding influence from an electro-magnetic field, a difference in ground-fault electric potential, a magnetic filed or the like. Optical modulation element 4 is connected to light source 3 or optical detector 9 via optical fibers 5 or 6. Then, a light exiting from light source 3 is modulated by optical modulation element 4. The modulated light signal is received by optical detector 9 and is converted to electricity. After that, it reconstructs the information related to sensor 7 on the ground by demodulator 10.
For such remote sensing technique, it is necessary to position optical modulation element 4 adjacent to sensor 7 in order to transmit the signal from sensor 7 without degrading it. Under the ground, the environment temperature rises about 1° C./30 m against the depth of a measuring point. Thus, because exploration is performed in a deep layer of 5 km for the underground resources exploration and the seismic layer exploration, the environment temperature of optical modulation element 4 becomes around 200° C. in some cases.
An optical modulator of normal configuration would have various problems in such environment temperature.
In particular, there are problems such as    (1) the rupture of a fiber due to degradation of the fiber jacket's material,    (2) an increase in optical insertion loss due to thermal degradation of each adhesive material which affixes constituent members to a case or to each other, and    (3) the rupture of a fiber due to a difference in linear expansion between a case and a fiber.    (4) Further, a so-called LN optical modulator as an optical modulation element has problems as below, the LN optical modulator forming a Mach-Zehnder type waveguide on the surface of an LiNb3 crystal, applying a modulating signal between electrodes being positioned adjacent to said waveguide, intensely modulating an input light and outputting the modulated light. Although it is necessary to apply a DC voltage between electrodes in order to set an operating point, internal electric field is generated due to said DC voltage and said internal electric field works in the direction of canceling the effect of said applied voltage. Therefore, it is necessary to apply bigger DC voltage in order to keep the operating point stable (DC drift phenomenon). Further, said DC drift is influenced a lot by the applied voltage amount and the environment temperature. It takes shorter to reach the upper limit of the applied DC voltage as the initial DC voltage gets bigger, or the temperature gets higher. Said time until the upper limit is an operation life and operating conditions are limited.    (5) Because the operating point shifts in accordance with a temperature (a temperature drift), operating conditions are limited in a hot temperature environment.